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United States Patent |
5,006,247
|
Dennison
,   et al.
|
April 9, 1991
|
Asymmetric porous polyamide membranes
Abstract
Asymmatric, porous polyamide membranes permeable to fluid flow comprising:
(A) a porous skin layer, and
(B) adjacent to the skin layer, an integral, porous support layer having at
least one region comprising a network of substantially parallel, hollow
tube-like structures, each of the tube-like structures being oriented such
that the longitudinal dimension of each tube-like structure is essentially
perpendicular to the skin layer;
wherein
the skin layer is relatively thin and dense compared to the support layer,
the pore diameters in the skin layer are relatively small compared to the
diameters of the tube-like structures in the support layer,
in membrances having support layers with more than one region, the
cross-sectional area of the tube-like structures is larger in regions
located farther from the skin layer, and
the polyamide is an aliphatic polyamide having a glass transition
temperature of less than 200.degree. C., and method of making the same.
These membranes are useful in ultrafiltration and microfiltration
processes.
Inventors:
|
Dennison; Kathleen A. (St. Paul, MN);
Kolcinski; Bruce E. (Birchwood Village, MN);
Krishnan; Subramanian (St. Paul, MN);
Russell; Patrise M. (Birchwood Village, MN)
|
Assignee:
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Minnesota Mining and Manufacturing Company (St. Paul, MN)
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Appl. No.:
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394229 |
Filed:
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August 15, 1989 |
Current U.S. Class: |
210/500.38; 264/45.1; 264/212 |
Intern'l Class: |
B01D 067/00; B01D 069/02; B01D 071/56 |
Field of Search: |
525/276,426
210/634,640,644,649-654,500.21,500.27,500.23,500.38
264/41,41.5,556,212,298,DIG. 48,DIG. 68
|
References Cited
U.S. Patent Documents
3615024 | Oct., 1971 | Michaels | 210/490.
|
3876738 | Apr., 1975 | Marinaccio et al. | 264/41.
|
3988245 | Oct., 1976 | Wang | 210/500.
|
4340479 | Jul., 1982 | Pall | 210/490.
|
4340480 | Jul., 1982 | Pall et al. | 210/490.
|
4340481 | Jul., 1982 | Mishiro et al. | 210/500.
|
4482514 | Nov., 1984 | Schindler et al. | 264/41.
|
4627992 | Dec., 1986 | Badenhop et al. | 427/244.
|
4629563 | Dec., 1986 | Wrasidlo | 210/500.
|
4666991 | May., 1987 | Matsui et al. | 525/426.
|
4722795 | Feb., 1988 | Gohl et al. | 210/500.
|
4818452 | Apr., 1989 | Kneifel et al. | 264/41.
|
Foreign Patent Documents |
037730B1 | ., 0000 | EP.
| |
0247596A2 | ., 0000 | EP.
| |
58094863 | ., 0000 | JP.
| |
63296940 | ., 0000 | JP.
| |
Other References
Kirk-Othmer Encyclopedia of Chemical Technology, 34d ed., vol. 18, John
Wiley & Sons, 1982, pp. 350-353.
DuPont Brochure No. E-26345.
Monsanto Brochure No. 8-1117,0880-02.
Celanese "Bulletin No. N1A", Brochure No. 10M/583-5M85.
|
Primary Examiner: Sever; Frank
Attorney, Agent or Firm: Griswold; Gary L., Kirn; Walter N., Maki; Eloise J.
Claims
We claim:
1. An asymmetric, porous polyamide membrane permeable to fluid flow
comprising:
(A) a porous skin layer, and
(B) adjacent to the skin layer, an integral, porous support layer having at
least one region comprising a network of substantially parallel, hollow
tube-like structures, each of the tube-like structures being oriented such
that the longitudinal dimension of each tube-like structure is essentially
perpendicular to the skin layer;
wherein:
the skin layer is relatively thin and dense compared to the support layer,
the pore diameters in the skin layer are relatively small compared to the
diameters of the tube-like structures in the support layer,
in membranes having support layers with more than one region, the
cross-sectional area of the tube-like structures is larger in regions
located farther from the skin layer, and
the polyamide is an aliphatic polyamide having a glass transition
temperature of less than 200.degree. C.
2. The membrane of claim 1 wherein each tube-like structure in a particular
region of the support layer has approximately the same cross-sectional
area.
3. The membrane of claim 1 having an asymmetric ratio greater than 10.
4. The membrane of claim 1 having an average pore diameter in the skin
layer of 0.001 to 10 .mu.m.
5. The membrane of claim 1 wherein the polyamide is selected from the group
consisting of
(A) polyamides having the formulas:
##STR2##
wherein each R group may be independently selected from the group
consisting of H, and alkyl groups having 1 to 4 carbon atoms, and each
R.sup.1 and R.sup.2 or R.sup.3 group may be independently selected from
the group consisting of linear or branched aliphatic or cycloaliphatic
moieties or combinations thereof, wherein said R.sup.1, R.sup.2, and
R.sup.3 groups can also contain hetero atoms, or
(B) blends thereof.
6. The membrane of claim 5 wherein all R.sup.1 groups in any particular
polymer (I) are the same, and all R.sup.2 groups in any particular polymer
(I) are the same, and all R.sup.3 groups in any particular polymer (II)
are the same.
7. The membrane of claim 5 wherein R.sup.1, R.sup.2 and R.sup.3 groups
contain from 1 to 12 carbon atoms.
8. The membrane of claim 1 wherein the polyamide is selected from the group
consisting of nylon 6, nylon 6,6, nylon 6,9, copolymers thereof, and
blends thereof.
9. The membrane of claim 1 having an air flux rate of 2.5 to 75
L/kPa(sec)m.sup.2.
10. The membrane of claim 1 in the form of a sheet.
11. The membrane of claim 1 in the form of a hollow fiber.
12. A supported asymmetric membrane comprising the membrane of claim 1 and
a preformed, porous, support layer having two parallel surfaces.
13. The membrane of claim 12 wherein the support layer is selected from the
group consisting of non-woven web, woven cloth and paper.
14. The membrane of claim 12 wherein the non-woven web is a spun-bonded,
nylon 6,6 non-woven web.
15. The membrane of claim 1 prepared by a process comprising the steps of:
(A) preparing a casting solution comprising an aliphatic polyamide, having
a glass transition temperature less than 200.degree. C., dissolved in an
alcohol/salt solution;
(B) coating a film of preselected thickness of the casting solution onto
the surface of a casting substrate;
(C) exposing the resulting film to a non-solvent liquid bath thereby
precipitating the polyamide and forming the porous, asymmetric polyamide
membrane; and
(D) recovering the membrane from the bath and drying it.
16. The membrane of claim 15 wherein the process of preparing the membrane
further comprises a step after step (B) and before step (C) in which the
film of casting solution is exposed to air for a period of up to 120
seconds.
17. The membrane of claim 15 wherein said alcohol is a lower alkanol or a
blend of lower alkanols.
18. The membrane of claim 17 wherein said alcohol is selected from the
group consisting of methanol, ethanol, propanol, butanol, and mixtures of
ethanol and methanol, ethanol and propanol, and methanol and propanol.
19. The membrane of claim 15 wherein the salt is selected from the group
consisting of anhydrous calcium chloride, calcium chloride dihydrate,
calcium iodide tetrahydrate, calcium nitrate tetrahydrate, calcium
salicylate, lithium chloride and blends thereof.
20. The membrane of claim 15 wherein the casting solution comprises 10 to
50 weight percent salt, 47 to 76 weight percent alcohol, and 3 to 14
weight percent polyamide.
21. The membrane of claim 12 prepared by a process comprising the steps of:
(A) preparing a casting solution comprising an aliphatic polyamide, having
a glass transition temperature less than 200.degree. C., dissolved in an
alcohol/salt solution;
(B) coating a film of preselected thickness of the casting solution onto
one or both parallel surfaces the porous, preformed support;
(C) placing the coated, preformed support on a casting substrate;
(D) immersing the coated, preformed support and casting substrate in a
non-solvent liquid bath thereby precipitating the polyamide and forming
the supported, porous, asymmetric polyamide membrane; and
(E) recovering the supported membrane from the bath and drying it.
22. The membrane of claim 21 wherein the process of preparing the membrane
further comprises a step after step (C) and before step (D) in which the
coated, preformed support is exposed to air for a period of up to 120
seconds.
23. The membrane of claim 21 wherein the porous preformed support is
selected form the group consisting of non-woven webs, woven fabrics, and
paper.
24. The membrane of claim 11 prepared by a process comprising the steps of:
(A) preparing a casting solution comprising an aliphatic polyamide, having
a glass transition temperature less than 200.degree. C., dissolved in an
alcohol/salt solution;
(B) extruding a hollow fiber of the casting solution through an annular
die;
(C) immersing the resulting fiber in a non-solvent liquid bath thereby
precipitating the polyamide and forming the porous, asymmetric, polyamide,
hollow fiber membrane; and
(D) recovering the membrane from the bath and drying it.
25. The membrane of claim 24 wherein the process of preparing the membrane
further comprises a step after step (B) and before step (C) in which the
fiber of casting solution is exposed to air for a period of up to 120
seconds.
26. A process for making a porous asymmetric membrane comprising the steps
of:
(A) preparing a casting solution comprising an aliphatic polyamide, having
a glass transition temperature less than 200.degree. C., dissolved in an
alcohol/salt solution;
(B) coating a film of preselected thickness of the casting solution onto
the surface of a casting substrate;
(C) exposing the resulting film to a non-solvent liquid bath thereby
precipitating the polyamide and forming the porous, asymmetric polyamide
membrane; and
(D) recovering the membrane from the bath and drying it.
27. The process of claim 26 further comprising a step after step (B) and
before step (C) in which the film of casting solution is exposed to air
for a period of up to 120 seconds.
28. A process for preparing a supported, porous asymmetric membrane
comprising the steps of:
(A) preparing a casting solution comprising an aliphatic polyamide, having
a glass transition temperature less than 200.degree. C., dissolved in an
alcohol/salt solution;
(B) coating a film of preselected thickness of the casting solution onto
one or both parallel surfaces of a porous, preformed support;
(C) placing the coated, preformed support on a casting substrate;
(D) immersing the coated, preformed support and casting substrate in a
non-solvent liquid bath thereby precipitating the polyamide and forming
the supported, porous, asymmetric polyamide membrane; and
(E) recovering the supported membrane from the bath and drying it.
29. The process of claim 28 further comprising a step after step (C) and
before step (D) in which the coated, preformed support is exposed to air
for a period of up to 120 seconds.
30. A process of preparing a porous, asymmetric hollow fiber member
comprising the steps of:
(A) preparing a casting solution comprising an aliphatic polyamide, having
a glass transition temperature less than 200.degree. C., dissolved in an
alcohol/salt solution;
(B) extruding a hollow fiber of the casting solution through an annular
die;
(C) immersing the resulting fiber in a non-solvent liquid bath thereby
precipitating the polyamide and forming the porous, asymmetric, polyamide,
hollow fiber membrane; and
(D) recovering the membrane from the bath and drying it.
31. The process of claim 30 further comprising a step after step (B) and
before step (C) in which the fiber of casting solution is exposed to air
for a period of up to 20 seconds.
Description
FIELD OF THE INVENTION
This invention relates to porous asymmetric polyamide membranes useful in
microfiltration and ultrafiltration.
BACKGROUND OF THE INVENTION
Canadian Patent No. 1,168,006 (Wrasidlo) discloses an asymmetric
ultrafiltration membrane having a skin layer supported by an "open
honeycomb structure". The specification states that only polymeric
materials having glass transition temperatures of at least 200.degree. C.
would be suitable in the invention since only polymers having such
relatively high glass transition temperatures are sufficiently rigid to
form the honeycomb support structure. Of the polyamides, only
polyarylamides were specifically identified as useful. The specification
also states that the skin layer of the membranes has slit-like fissures
instead of substantially circular pores.
U.S. Pat. No. 4,629,563 (Wrasidlo) discloses asymmetric membranes which may
be used as ultrafilters or microfilters. The membranes can have a skin
layer and a porous, asymmetric, reticulated support layer. The membranes
described in the '563 patent can be made of polyamide, however, only
polyhexamethylene terephthalamide, an araliphatic polyamide, is
specifically disclosed. The membranes are formed by casting a polymer dope
while that dope is in an unstable liquid dispersion condition.
U.S. Pat. No. 4,627,992 (Badenhop et al.) describes membranes formed from
aromatic polyamide resins. The membranes described in the '992 patent can
be symmetric or asymmetric and can be produced using a solution of the
polyamide in an aprotic solvent.
U.S. Pat. No. 4,340,481 (Mishiro et al) describes a hollow fiber membrane
having a three-dimensional net-like structure of fine pore passages. In
Example 8, one such hollow fiber membrane is prepared from a casting
solution containing 18 weight percent polyamide, methyl alcohol, water and
calcium chloride dihydrate using a 1:1 methanol/water coagulation bath.
U.S. Pat. No. 4,722,795 (Gohl et al.) describes asymmetric, self-supporting
membranes, useful for ultrafiltration, which may be in the form of a flat
sheet, tube, or hollow fiber. The patent states that suitable membrane
materials are polymers which are soluble in polar, non-protonic organic
solvents such as dimethylsulfoxide. While the patent states that
polyamides are preferred for use in forming the membranes, only an
aromatic polyamide is specifically disclosed.
U.S. Pat. No. 3,876,738 (Marinaccio et al.) describes a process for making
microporous membranes in which a film-forming polymer, e.g. nylon, is
dissolved in a solvent system, e.g., a formic acid solution, the resulting
solution is cast to form a film, and the film is quenched in a non-solvent
bath, e.g. water/salt or alcohol/salt solutions, to form a microporous
membrane. This patent does not state that these membranes are asymmetric.
U.S. Pat. No. 4,482,514 (Schindler et al.) describes a process for
preparing ultrafiltration membranes using a formic acid solution of
polyamide containing about 1 to 7% polyethylene glycol. The patent states
that the membrane possesses an ultrafiltration skin layer and a backing
layer in which the pore size increases with the distance from the
ultrafiltration skin. FIG. 2 of the patent depicts a cross-sectional view
of a membrane having a gradually increasing pore size.
Japanese Kokai Application JP 58094863, laid open June 6, 1983, describes
polyamide membranes having a rough surface and a smooth surface. It
describes the process of preparing one such membrane which includes
preparation of a 10 to 40 weight percent solution of nylon 6,6 dissolved
in formic acid mixed with calcium chloride and water, casting of the
resulting solution, and intermittent coagulation, e.g. by repeated dipping
of the cast membrane in a coagulation bath, of the cast film to form an
uneven surface. The disclosed pore sizes range from 0.05 to 500 .mu.m on
the rough surface and 1 to 1000 .mu.m on the smooth surface.
U.S. Pat. No. 3,615,024 (Michaels) describes anisotropic membranes which
can be formed from polyamides such as polyhexamethylene adipamide and
other such polyamides known as "nylon", however, no such membranes are
exemplified.
U.S. Pat. Nos. 4,340,480 and 4,340,479 (Pall et al.) disclose a process for
preparing skinless liquofilic alcohol-insoluble polyamide microfiltration
membranes from polyamide having a ratio of methylene to amide groups of
about 7 to 1 to about 12 to 1. The patents disclose that membranes
prepared according to the described process have pores of uniform
diameter. The patents also state that their membranes can have "tapering
pores" but they do not describe highly asymmetric membranes.
SUMMARY OF THE INVENTION
This invention provides an asymmetric, porous polyamide membrane permeable
to fluid flow comprising:
(A) a porous skin layer, and
(B) adjacent to the skin layer, an integral, porous support layer (i.e., a
support layer formed in the same operation or step and having
substantially the same composition as the skin layer) having at least one
region comprising a network of substantially parallel, hollow tube-like
structures, each of the tube-like structures being oriented such that the
longitudinal dimension of each tube-like structure is essentially
perpendicular to the skin layer;
wherein:
the skin layer is relatively thin and dense compared to the support layer,
the pore diameters in the skin layer are relatively small compared to the
diameters of the tube-like structures in the support layer, and
in membranes having support layers with more than one region, the
cross-sectional area of the tube-like structures is larger in regions
located farther from the skin layer, and
the polyamide is an aliphatic polyamide having a glass transition
temperature of less than 200.degree. C.
This invention further provides supported asymmetric membranes, i.e.,
membranes comprising the polyamide membrane described above and a
preformed, porous, support layer, e.g. non-woven web, woven cloth or
paper, having two parallel surfaces.
This invention also provides a method of preparing the asymmetric and
supported asymmetric membranes described above comprising the steps of:
(a) preparing a casting solution comprising the polyamide dissolved in an
alcohol/salt solution;
(b) coating a film of preselected thickness of the casting solution onto
the surface of a casting substrate (e.g. a TEFLON-coated surface, glass,
or polyethylene terephthalate) or coating a film of preselected thickness
of the casting solution onto one or both of the parallel surfaces of a
preformed, porous support material and placing the resulting coated
preformed support on a casting substrate, or
(c) extruding a hollow fiber of the casting solution through an annular
die;
(d) immersing the resulting film, coated preformed support, or hollow fiber
in a liquid non-solvent bath thereby precipitating the polyamide and
forming an asymmetric polyamide membrane or asymmetric, supported
polyamide membrane, and recovering the membrane or supported membrane from
the non-solvent bath and drying it.
The membranes of this invention can be useful in microfiltration or
ultrafiltration separation processes depending upon the pore size of the
porous skin layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is reproduced from a scanning electron photomicrograph showing a 420
.times.s magnification of a cross-sectional view of one of applicants'
membranes. This membrane was formed from a casting solution comprising 5
weight percent, based on total casting solution weight, of polyamide.
FIG. 2 is reproduced from a scanning electron photomicrograph showing a
11,700 .times.s magnification of the top surface of the porous skin layer
of the membrane shown in FIG. 1.
FIG. 3 is reproduced from a scanning electron photomicrograph showing a 116
.times.s magnification of the bottom surface of the porous support layer
of the membrane shown in FIG. 1.
FIG. 4 is reproduced from a scanning electron photomicrograph showing a 590
.times.s magnification of the bottom surface of the porous support layer
of the membrane shown in FIG. 3.
FIG. 5 is reproduced from a scanning electron photomicrograph showing a
3500 .times.s magnification of the top surface of a porous skin layer
formed from a casting solution comprising 9 weight percent, based on total
casting solution weight, of polyamide.
FIG. 6 is reproduced from a scanning electron photomicrograph showing a 250
.times.s magnification of a cross-sectional view of the membrane shown in
FIG. 5.
FIG. 7 is reproduced from a scanning electron photomicrograph showing a
5000 .times.s magnification of the top surface of a porous skin layer
formed from a casting solution comprising 12 weight percent, based on
total casting solution weight, of polyamide.
FIG. 8 is reproduced from a scanning electron photomicrograph showing a 350
.times.s magnification of a cross-sectional view of the porous membrane
shown in FIG. 7.
DETAILED DESCRIPTION OF THE INVENTION
Some of the asymmetric membranes of this invention have support layers
comprising two or more regions or layers each of which comprises a
three-dimensional network of substantially parallel, hollow tube-like
structures oriented such that the longitudinal dimension of each tube-like
structure is essentially perpendicular to the skin layer. Generally, each
tube-like structure in a particular region of the support layer will have
approximately the same cross-sectional area. However, the tube-like
structures in adjacent support layer regions may have significantly
different cross-sectional areas. This may result in the support layer
appearing to be comprised of distinct, discontinuous regions or layers
each comprising tube-like structures of approximately the same
cross-sectional area. When adjacent regions are comprised of tube-like
structures having similar cross-sectional areas, the support layer may
appear to be comprised of tapered tube-like structures whose
cross-sectional area increases with increasing distance from the skin
layer.
The polyamides useful in preparing the asymmetric membranes of this
invention have glass transition temperatures below 200.degree. C.,
preferably below 100.degree. C., and most preferably below 60.degree. C.
The polyamides useful in this invention are aliphatic, that is, they can
be selected from the group consisting of aliphatic polyamides, which
includes cycloaliphatic polyamides. Blends of polyamides are also useful
in this invention.
Some classes of the polyamides useful in this invention can be represented
by the following general formulas:
##STR1##
wherein each R group may be independently selected from the group
consisting of H, and alkyl groups having 1 to 4 carbon atoms, and each
R.sup.1 and R.sup.2 or R.sup.3 group may be independently selected from
the group consisting of linear or branched aliphatic or cycloaliphatic
moieties or combinations thereof. Generally in any particular polymer (I),
all R.sup.1 groups will be the same and all R.sup.2 groups will be the
same, and in any particular polymer (II), all R.sup.3 groups will be the
same. Generally R.sup.1, R.sup.2, and R.sup.3 groups will have from 1 to
12 carbon atoms, and preferably 1 to 10. R.sup.1, R.sup.2, and R.sup.3
groups can also contain hetero atoms such as oxygen, nitrogen, and sulfur.
Generally the polyamides useful in this invention will have weight average
molecular weights in excess of about 10,000 and can be prepared according
to conventional methods such as those described in Kirk-Othmer
Encyclopedia of Chemical Technology, 3rd Ed., Vol. 18, John Wiley & Son
1982, pp. 350-353. Many of the polyamides useful in this invention are
known and are commercially available, e.g., nylon 6,6 (i.e.,
poly(hexamethylene adipamide), nylon 6,9, and nylon 6 (i.e.,
polycaprolactam). Nylon 6,6 is commercially available from E.I. DuPont de
Nemours Company as ZYTEL 101 or 120, described in DuPont brochure no.
E-26345, from Monsanto Corporation as VYDYNE 21, described in Monsanto
brochure no. 8-1117-0880-92, and from Celanese Corporation as 1001-1 or
1200-1, described in Celanese "Bulletin NlA" brochure no. 10M/583-5M85.
Nylon 6 is commercially available from Badische Corporation as B-300 and
B-4407, and from Nylon Corporation of America as NYCOA 471, 466, and 589.
Nylon 6,9 is commercially available from Aldrich Chemical Company and from
Monsanto Corporation as VYDYNE 60H and 602M and Nylon 6,6/6 copolymer
available from Monsanto Corporation as VYDYNE 80X.
The casting solution used to make the asymmetric membranes of this
invention is prepared by dissolving one or more of the polyamides
described above in casting solvent. A casting solvent is a solution of
salt dissolved in a lower (i.e., C1 to C4) alcohol or mixture of lower
alcohols. The selection of alcohol, salt, and level of salt is largely
determined by the solubility of the polyamide in the casting solvent.
Representative examples of lower alcohols and lower alcohol mixtures
useful in this invention are lower alkanols and mixtures of lower
alkanols, e.g., methanol, ethanol, propanol, butanol, and
ethanol/methanol, ethanol/propanol, and methanol/propanol mixtures.
Methanol, ethanol, and mixtures containing at least 50 weight percent of
methanol or ethanol are more preferred because a number of commonly
available salts are soluble in them. The salt used to make the casting
solution should increase the solubility of the polyamide in the alcohol.
Representative salts include anyhdrous calcium chloride, calcium chloride
dihydrate, calcium iodide tetrahydrate, calcium nitrate tetrahydrate,
calcium salicylate and lithium chloride and blends thereof. Preferably
calcium chloride dihydrate is used to make the casting solvent.
The casting solution is a homogeneous solution at the time and temperature
of casting the membrane. Preferably, it is a stable solution, i.e., does
not phase separate upon standing at room temperature. The casting solution
should also have a viscosity low enough to permit stirring and coating of
the solution on a casting substrate or on a preformed membrane support.
The solution viscosity will depend to some extent on the type, molecular
weight, and concentration of polyamide used to make the solution. For
example, a particular casting solution containing 8 weight percent of
nylon 6,6 will have a viscosity of about 5000 cp at room temperature while
a 5 weight percent solution will have a viscosity of about 1200 cp at room
temperature. Casting solution viscosities below about 500 cp at room
temperature are generally not preferred.
Typically the casting solution contains 10 to 50 weight percent of salt,
based on total casting solution weight, and preferably 35 weight percent.
The alcohol generally comprises 47 to 76 weight percent of the casting
solution, based on total casting solution weight, and preferably about 60
weight percent. Generally the casting solution comprises about 3 to 14
weight percent of polyamide, based on the total weight of the casting
solution. The optimum polyamide concentration depends on the molecular
weight of the polymer and evaporation time used. However, the desired
polyamide must be soluble at a level of at least about 3 weight percent
because concentrations of less than about 3 weight percent will not result
in the formation of a membrane having the desired structure.
A preferred casting solution can be prepared by mixing 35 parts by weight
of calcium chloride dihydrate with 65 parts by weight of methanol,
refluxing the mixture until the calcium chloride dihydrate has completely
dissolved, mixing 4 to 7 parts by weight of nylon 6,6 with the
salt/alcohol solution, and refluxing the resulting mixture until the nylon
has completely dissolved.
Optionally, the casting solution can also contain non-interferring,
conventional casting solution additives. The relative amounts and types of
additives should be selected so as not to adversely affect final membrane
structure. One such additive is non-ionic surfactant, e.g. TRITON-X 100
commercially available from Rohm & Haas Corporation or TWEEN 20
commercially available from ICI Americas, Incorporated. Blends of
non-ionic surfactants may also be used. Generally the casting solution
will comprise less than 1 weight percent surfactant and more typically
less than 0.5 weight percent. The casting solution may also comprise small
amounts of pore formers, swelling agents, and plasticizers. The casting
solution may further comprise non-solvent, e.g. water, or blends of
non-solvents. Generally, the effect of added non-solvent on the final
membrane structure is relatively small, but typically an increasing amount
of non-solvent in the casting solution results in larger pores in the
membrane's skin layer. The maximum amount of additives that can be added
to the casting solution is limited by the solubility of the polyamide in
the resulting solution and may depend on the salt concentration in the
casting solution and the type of salt used in the casting solution.
Generally, the higher the salt concentration the greater the amount of
water that can be added to the casting solution. However, the maximum
amount of water that can be added to the casting solution is reduced by
any amount of water that was initially complexed to the salt. For example,
the maximum amount of water that can be added to a casting solution
comprising methanol and calcium chloride is about 0.8 g of water per 1 g
of calcium chloride.
The casting solution can be formed into a sheet or film by coating or
depositing on the desired area of the exposed surface of a casting
substrate the desired thickness of casting solution. The casting
substrates useful in this invention include many of the substrates known
in the art. For example, the substrates include, glass, glass-coated
materials, stainless steel, poly(ethylene terephthalate), polypropylene,
poly(tetrafluoroethylene)-coated materials, and silicone coated paper or
belts. Conventional casting methods and equipment may be used such as
knife coaters, slot-fed knife coaters, and notch bar coaters. The casting
solution may also be coated directly onto a porous, preformed support
having two parallel surfaces to form one of the supported or composite
membranes of this invention. Representative examples of such preformed
supports are non-woven fibrous webs, woven fabrics or paper. The casting
solution is applied to one or both of the parallel surfaces. Casting the
membrane on such a support can provide additional strength to the
membrane. The preformed support must have a very porous or open structure
so that it does not restrict the flow of fluids through the membrane. That
is, the average pore size of the support should be at least as large as
the biggest pore in the membrane. In order to facilitate the formation of
the membrane and reduce the number of pinholes in the membrane, the
preformed support should be made of a material which can be wet by the
casting solution. Generally, a preformed support comprising polyamide
material will be more easily wet by the casting solution.
When a hollow fiber membrane is desired, the casting solution can be
extruded through an annular die. Optionally, air or liquid in which the
polyamide is insoluble can flow through the lumen of the fiber. The liquid
need not be the same as the non-solvent used in the non-solvent bath.
Conventional extrusion methods and equipment may be used to prepare hollow
fiber membranes of this invention.
The solution can be cast or extruded to a wet thickness about 0.05 to 0.51
mm (0.002 to 0.020 in) thick. This generally will result in a dry membrane
about 0.025 to 0.25 mm (0.001 to 0.010 in) thick. Preferably, the solution
is cast to a wet thickness of about 0.10 to 0.18 mm (0.004-0.007 in) which
generally results in dry membranes between 0.051 to 0.152 mm (0.002 to
0.006 in) thick.
After depositing the casting solution on the substrate, the coated
substrate may optionally be exposed to air for drying. Exposure to air can
affect the thickness and porosity of the skin layer of the membrane. The
longer the membrane is exposed to air, the less porous the skin layer of
the membrane will be. Exposure times of up to 120 seconds may be used,
however, an exposure time about 30 seconds or less, e.g. 5 to 15 seconds,
is preferred.
After the optional drying step described above, the still-wet membrane is
exposed to a non-solvent, such as water or water vapor, to precipitate the
polyamide to form the membrane. The preferred non-solvent comprises water
or water and water vapor. Exposure to non-solvent only in the form of
vapor does not yield membranes having applicants' characteristic membrane
structure. Hence if non-solvent vapor is to be used to precipitate the
polyamide, exposure to the vapor must be short, e.g., 10 min. or less, and
such exposure must be followed by immersion of the membrane in a liquid
non-solvent bath. When water vapor is used as one of the non-solvents, the
humidity in the chamber should be kept above about 50 percent relative
humidity, and most preferably should be 90 to 100 percent relative
humidity. However, since it is simpler to use only one precipitation step,
immersion of the membrane in a liquid non-solvent bath is the preferred
method of exposing the wet membrane to the non-solvent.
Some of the liquid non-solvent baths useful in this invention comprise a
major amount of water and a minor amount of a polar organic solvent.
Suitable polar organic solvents include the lower (i.e. having one to four
carbon atoms) alkanols, e.g. isopropanol, the lower ketones, e.g., acetone
and methylethyl ketone, lower ethers, e.g., tetrahydrofuran, lower
nitriles, e.g., acetonitrile, and lower amides, e.g., dimethyl formamide.
Acetone is particularly preferred. As much as 20 to 25 volume percent of
the total non-solvent bath may be polar organic solvent. However,
preferably the bath will contain less than about 20 volume percent, and
most preferably less than 15 volume percent of polar organic solvent.
The liquid non-solvent bath is generally maintained at room temperature.
Bath temperatures below room temperature tend to result in slower
coagulation or precipitation rates. The coated casting substrate or
extruded hollow fiber should be immersed in the bath and remain there for
at least 1 minute or until the membrane begins to float off the casting
substrate. The membrane may then be recovered from the bath and washed
with water.
A membrane may be dried after formation and recovery by any suitable means.
Drying with forced, heated air hastens drying time but promotes wrinkling
of the membrane if the membrane is not dimensionally secured during
drying. A wet membrane allowed to dry under ambient conditions releases
easily from polytetrafluoroethylene-coated substrate surfaces.
The flux rate through the membranes of this invention can be varied by
varying the percent porosity of the membrane (i.e., percent of the
membrane volume comprising voids), the asymmetric ratio (i.e., the ratio
is the average pore diameter on the bottom surface of the support layer
divided by the average pore diameter of the skin layer), and/or the
average pore diameter of the skin layer The flux rate can also be varied
by controlling the thickness of the membrane, and by using multiple
thicknesses of membrane. For example, a single layer or thickness of some
of the microfiltration membranes of this invention will have normalized
air flux rates of about 2.5 L/kPa(sec)m.sup.2 to about 75
L/kPa(sec)m.sup.2, and will have bubble points of about 0.5 to about 20
.mu.m.
The process variables having the greatest impact on the percent porosity of
the membrane, asymmetric ratio, and average pore diameter of the retentive
layer are:
(1) the concentration of the polyamide in the casting solution,
(2) the concentration of non-solvent in the liquid non-solvent bath, and
(3) the time the membrane is exposed to air between the casting and
precipitation steps. For example, a reduction of average pore diameter of
the skin or retentive layer may be achieved by increasing the
concentration of polyamide in the casting solution and/or increasing the
time of exposure to air between casting and precipitation steps. The
average pore diameter can be increased by increasing the amount of polar
organic solvent in the liquid non-solvent bath.
The average pore diameter of the skin layer of membranes of this invention
is selected based on the desired application, i.e. ultrafiltration or
microfiltration, of the membrane and can be controlled using the process
variables described above. Some of the membranes of this invention have
average skin layer pore diameters of about 0.001 .mu.m to 10 .mu.m, and
average pore diameters on the bottom surface of the support layer of about
20 to 500 .mu.m. Typically, the percent porosity of the membranes of this
invention will be greater than about 70 percent, and preferably greater
than 80 percent. The asymmetric ratio must be greater than 1, and
preferably is greater than about 10, e.g., about 10 to 100,000. Asymmetric
ratios of 2000 or more have easily been achieved using the process of this
invention. Large asymmetric ratios are desirable since a high degree of
membrane asymmetry can contribute to higher flux rates through the
membrane. However, membranes having large pore diameters on the bottom
surface of the support layer may be more likely to compact or compress
during use resulting in lower membrane flux rates. However, the degree of
compaction is also strongly dependent on membrane thickness. For example,
a nylon 6,6 membrane having a dry thickness of 0.25 mm may have an 87
percent flux decline at 345 kPa (as determined by the difference between
the initial flux and the steady state flux at the measured pressure). A
membrane having a 0.12 mm dry thickness may have a flux decline of 21
percent or less at 345 kPa, and a membrane having a 0.04 mm dry thickness
may have a flux decline of 10 percent or less at 345 kPa.
Objects and advantages of this invention are illustrated in the following
examples, in which the preparation of various membranes are described as
well as some tests thereof indicative of their utility.
EXAMPLE 1
An amount of calcium chloride dihydrate (230 g) was mixed with 500 mL of
methanol. The resulting mixture was refluxed overnight to yield a clear
solution. An amount (25 g) of nylon 6,6, available as CELANESE 1200-1
(avg. MW 135,000), was added to 475 g of the solution of calcium chloride
in methanol. This mixture was refluxed overnight to yield a clear casting
solution (containing 5 weight percent polyamide) which was allowed to cool
to room temperature before casting the membrane.
An amount (35 mL) of the casting solution was deposited onto a 35 cm
.times. 50 cm glass plate and spread to a wet thickness of 254 .mu.m to
305 .mu.m using a glass plate with shims parallel to the edges of the
plate. The casting plate was immediately immersed in a non-solvent bath
consisting of 1900 mL of water and 100 mL of acetone held at
22.degree.-24.degree. C. The plate remained immersed in the bath for about
1 minute after which the membrane was removed from the bath, blotted with
paper towels, and was laid on a TEFLON-coated sheet and allowed to dry,
skin side up, at ambient temperature for about 3 hours. The dry thickness
of the membrane was about 51 to 64 .mu.m.
Bubble points and normalized air flow rates were measured on the membrane.
Both tests were performed on 47 mm diameter disks die-punched from a
pinhole-free area of the membrane. The air flow rate measurements were
carried out using an apparatus having a membrane sample mounted in a 47 mm
diameter, stainless steel filter holder from Fischer Scientific, Inc. The
membrane sample was mounted such that the skin layer faced upstream.
Nitrogen gas was applied to the upstream side of the membrane until a
standard flow rate of 14.85 standard liters/min. was measured on a
rotameter located on the downstream side of the membrane. When this flow
rate was achieved, the pressure drop across the membrane was measured
using an Ashcroft "brazed" pressure gauge. The air flow rate measurement
provides an indication of the resistance of the membrane to the transport
of fluids through the membrane. The normalized air flow rate was then
calculated using the following formula:
##EQU1##
where the pressure drop is in kPa and the filter area is in m.sup.2. The
membrane was found to have normalized air flow of 8.34
L/kPa(sec)(m.sup.2).
The bubble point was determined using an apparatus similar to that used to
measure the air flow rate. The bubble point was measured on a sample of
the membrane throughly wet with LUBINOL mineral oil (having a mathematical
constant of 14.4 which is commercially availiable from Kalipharmo, Inc.)
mounted in the filter holder such that the skin side of the membrane faced
upstream. A tube extended from the downstream side of the filter holder
into a reservoir filled with soapy water. The pressure on the upstream
side of the membrane was then increased by slowly increasing the flow of
nitrogen gas to the upstream side of the membrane until bubbles first
appeared in the soapy water. The pressure at which bubbles first appeared
was identified as the bubble point. Pore size was then be correlated to
the bubble point pressure using the following formula:
Pore size (.mu.m)=2.09/ pressure (kPa)
The membrane had a bubble point of 0.87 .mu.m.
The membrane was also subjected to a latex particle challenge test. The
challenge test was performed using an aqueous solution of Dow polystyrene
particles having a mean diameter of 0.22 .mu.m. The solutions of latex
particles had an absorbance of about 0.190-0.200 (measured using a Beckman
Model 35 spectrophotometer using a visible light source at a wave length
of 420 nm). A pinhole-free sample of membrane was mounted in an AMICON
Model 12, 25 mm filter assembly operated in a straight filtration, no
recirculation mode. Each of the latex particle solutions was filtered
through the membrane. The filtrate or permeate passing though the membrane
was collected and its absorbance was measured using the same
spectrophotometer and light source described above. The percent of
particles retained by the membrane was then calculated using the following
fomula:
##EQU2##
The membrane retained 99.5 percent of latex particles having a mean
diameter of 0.22 .mu.m illustrating the membrane's usefulness for
microfiltration processes.
Scanning electron photomicrographs of this membrane revealed that the
membrane possessed the characteristic structure of the membranes of this
invention. Scanning electron photomicrographs were also used to estimate
the average pore diameters. Photomicrographs showed that the average pore
diameter of the skin layer of the membrane was about 0.1 .mu.m and that
the average pore diameter on the bottom surface of the support layer was
about 100 to 200 .mu.m. Thus, the membrane had an asymmetric ratio of
about 1000:1 to about 2000:1.
EXAMPLES 2-3
The procedure described in Example 1 was repeated except that the casting
solution contained 5.5 weight percent of a Nylon 6 (commercially available
as NYCOA 471), and the acetone concentration in the non-solvent bath was
either 5 or 15 weight percent. Air flow rate measurements were carried out
according to the procedure described in Example 1. The membranes had
normalized air flow rates of 36.2 and 145 L/kPa(sec)(m.sup.2),
respectively.
EXAMPLES 4-14
The procedure described in Example 1 was repeated except that various types
and amounts of the polar organic solvent used in the non-solvent bath were
used. Air flow rates and bubble points were determined using the
procedures described in Example 1. The types and amounts of polar organic
solvent, and the properties of the resulting asymmetric membranes are
summarized in Table 1.
TABLE 1
______________________________________
Effect of Immersion Bath Composition
Bath Composition
Normalized
Bubble
(volume water/
air flow point
Example
Solvent volume solvent)
(L/kPa(sec)m.sup.2)
(.mu.m)
______________________________________
4 DMF 95/5 33.60 3.65
5 DMF 90/10 44.74 2.06
6 DMF 85/15 74.64 3.08
7 DMF 80/20 74.64 3.20
8 MEK 95/5 37.34 2.1
9 MEK 90/10 74.64 4.0
10 IPA 95/5 43.51 2.64
11 IPA 90/10 43.51 5.0
12 IPA 85/15 62.19 9.6
13 ACN 95/5 41.48 3.8
14 ACN 90/10 74.64 2.88
______________________________________
The data indicates that increasing the level of polar organic solvent in
the liquid non-solvent bath generally results in greater air flow rates.
EXAMPLES 15-16
The procedure of Example 1 was repeated except that a 5 weight percent
solution of a nylon 6 commercially available as NYCOA 471 was used as the
casting solution in Example 15, and 4.5 weight percent solution of the
nylon 6 was used in Example 16. In Comparative Examples C1 and C2, a
double thickness of each of the membranes of Examples 15 and 16,
respectively, were tested. The membranes were arranged so that their skin
layers were in a face-to-face orientation. The normalized air flow rates
and bubble points of the resulting asymmetric membranes and their double
thicknesses were measured according to the procedures described in Example
1. The membranes used, number of thicknesses, normalized air flow rates,
and bubble points are summarized in Table 2.
TABLE 2
______________________________________
Effect of Multiple Thicknesses
Normalized
Number of Flow Rate Bubble Point
Example thicknesses (L/kPa(sec)m.sup.2)
(.mu.m)
______________________________________
15 1 4.67 3.2
C1 2 2.42 0.65
16 1 7.81 7.0
C2 2 3.87 1.3
______________________________________
The data shows that while doubling the thickness of the membranes reduces
the bubble point of the membrane, it also reduces the normalized air flow
rate. This illustrates that a membrane of this invention can be sandwiched
or layered to vary the flow rates and bubble point of the final membrane.
EXAMPLE 17
The procedure of Example 1 was repeated except that the coated casting
substrate was immersed in a 95:5 volumetric mixture of water and methyl
ethyl ketone. The resulting membrane was 51 .mu.m thick and had normalized
air flow of 145 L/kPa(sec)m.sup.2 and a bubble point of 5 .mu.m. A double
thickness of this membrane had a normalized flow of 75 L/kPa(sec)m.sup.2
and a bubble point of about 2.5 .mu.m. A triple sandwich has a normalized
flow rate of greater than about 36 L/kPa(sec)m.sup.2 and a bubble point of
1.2 .mu.m. This illustrates that a membrane of this invention can be
sandwiched or layered to vary the flow rates and bubble point of the final
membrane.
EXAMPLE 18
This example illustrates the use of a mixture of polyamides to form one of
the membranes of this invention.
A 5.0 weight percent polyamide casting solution was prepared using a 50/50
weight percent mixture of nylon 6 (commercially available as NYCOA 589)
and nylon 6,6, (commercially available as CELANESE 1200-1) and a 37 weight
percent calcium chloride dihydrate solution in methanol. The resulting
mixture was refluxed about 12 hours to yield a generally clear and stable
solution.
This casting solution was then spread on a polyethylene terephthalate
(i.e., PET) casting substrate using 10.2 cm Gardner knife coater set at a
clearance of 0.25 mm. The coated casting substrate was then exposed to
ambient air for about 15 seconds, and immersed in a non-solvent bath of
deionized water. The coated casting substrate remained immersed in the
bath for about 1 to 2 minutes until the membrane began to separate from
the casting substrate. The membrane was recovered from the bath, blotted
dry with paper towels, placed on paper towels, skin side up, and allowed
to dry at room temperature for about 12 hours. Photomicrographs indicated
that the membrane possessed the characteristic structure of the membranes
of this invention, and that the average pore diameter in the skin or
retentive layer of the membrane was about 2 .mu.m and the average pore
diameter on the bottom surface of the support layer was about 60 to 80
.mu.m.
EXAMPLES 19-20
The following examples illustrate the use of surfactants in the membrane
casting solution.
The procedure described in Example 18 was repeated except that only the
nylon 6,6 was used to prepare the casting solution instead of the nylon 6
and nylon 6,6 blend and about 16 mg of TRITON-X 100 was added to about 5 g
of the casting solution and the mixture was agitated. The resulting
mixture was then deposited on a PET casting substrate and spread according
the the procedure described in Example 18. Scanning electron
photomicrographs of the cross-sections of resulting membrane indicated
that the membrane possessed the asymmetric structure characteristic of the
membranes of the this invention.
This procedure was repeated using about 16 mg of TWEEN 20. canning electron
photomicrographs of the resulting membrane indicated that this membrane
also possessed the asymmetric structure of the membranes of this
invention.
EXAMPLE 21
This example illustrates the preparation of one of the supported membranes
of this invention.
A casting solution was prepared according to the procedure of Example 18
except that the 5 weight percent casting solution was prepared using only
the nylon 6,6 described in Example 18. The casting solution was then
deposited on one surface of a nylon 6,6, spun-bonded, non-woven web
(commercially available from the James River Corporation as CEREX). The
coated, non-woven support was then placed on a PET casting substrate
coated side up. The coated non-woven support (still resting on the casting
substrate) was then exposed to ambient air for about 15 seconds, and
immersed in a non-solvent bath of water. The coated support and casting
substrate remained immersed in the bath for about 1 to 2 minutes after
which time, the coated non-woven support was recovered from the bath,
blotted dry with paper towels and was allowed to air dry, skin side up, at
ambient temperature for about 12 hours.
Scanning electron photomicrographs of a cross-section of the membrane
revealed that the membrane structure was somewhat interrupted by the
non-woven fibers, but that the membrane still possessed the asymmetric
structure characteristic of the membranes of this invention.
Photomicrographs of the top and bottom surfaces of the membrane indicated
that the average pore diameter of the skin layer was about 1 .mu.m and the
average pore diameter on the bottom surface of the support layer was about
20 to 50 .mu.m.
EXAMPLE 22
This example illustrates that greater water flow rates can be achieved
using Applicants' asymmetric membranes than can be achieved using a
commercially available, symmetric, nylon membrane.
A membrane was prepared using a 5 weight percent, nylon 6,6 (Celanese
1200-1) casting solution prepared using a 37 weight percent calcium
chloride dihydrate and methanol solution. The casting solution was spread
on a polyethylene terephthalate casting substrate using a notch bar coater
set at a gap of about 0.18 mm. The cast film was exposed to ambient air
for about 30 seconds and then immersed in a large non-solvent water bath.
The membrane remained immersed in the bath for about 1 to 2 minutes and
was then recovered, blotted dry with paper towels, and allowed to dry at
room temperature for about 12 hours. The resulting dried membrane was
about 0.06 mm thick and scanning electron microscopy revealed that the
membrane had skin layer pores of 0.5 .mu.m or less, and pore diameters on
the bottom surface of the support layer of 20 to 50 .mu.m.
The water flow rate through this membrane was measured using a 47 mm
diameter sample of the membrane die cut from a pinhole-free section of the
membrane. The membrane sample was mounted in an apparatus having a 47 mm
diameter, stainless steel, Fisher Scientific filter holder such that the
skin side of the membrane faced upstream. Water pressure was applied to
the upstream side from a pressurized reservoir and the flow of water
through the membrane measured with a rotameter located downstream from the
membrane. The pressure was held at 345 kPa (50 psig) until water flow
through the membrane became constant, at which point, the pressure was
decreased in 34.5 kPa (5 psig) increments and the flow rate measured at
each pressure.
The water flow rate was also measured on a nylon 6,6 membrane having a 0.22
.mu.m rating (commercially available from MSI Corporation as MAGNA
membranes). Scanning electron microscopy revealed that the MSI membrane
had surface pores on both surfaces of 2 .mu.m or less. The water flow rate
for Applicants' membrane was consistently about an order of magnitude
greater than that achieved by the MSI membrane over a pressure range of
34.5 to 207 kPa.
EXAMPLE 23
A membrane was cast from a 12 weight percent, nylon 6,6 (commercially
available as Celanese 1200-1) solution using the procedure described in
Example 18. except that the cast film was exposed to air for about 30
seconds. The solute rejection coefficient of the resulting membrane was
measured using a 0.5 wt. % solution of blue dextran (approximately
2,000,000 MW, available from Sigma Chemical Corporation) having an
absorbance of 1.30 at 260 nm, measured using a Hewlett-Packard 8451A Diode
Array Spectrophotometer. Ultrafiltration measurements were made on the
membrane using a 25 mm Amicon Stirred Ultrafiltration Cell. The permeate
volume removed during the experiment was sufficiently small relative to
the feed volume that the feed concentration did not appreciably change
during the experiment. The permeate was collected and its absorbance was
measured using the spectrophotometer described above. The rejection
coefficient was then determined using the equation:
rejection coefficient=1-C.sub.p /C.sub.f
where C.sub.f is the concentration of dextran in the feed solution and
C.sub.p is the concentration of dextran in the permeate. At a
transmembrane pressure of 276 kPa, the rejection coefficient was 0.95.
This indicates that the membrane functioned as an ultrafiltration
membrane.
EXAMPLES 24-29
The procedure of Example 18 was repeated except that (a) a 35 weight
percent solution of calcium chloride dihydrate dissolved in methanol was
used to prepare a 5.0 weight percent, nylon 6,6 (available from Celanese
as 1200-1) casting solution, and (b) various air exposure times were used.
The resulting membranes were studied using scanning electron microscopy.
Photomicrographs were used to estimate the average pore diameters in the
skin layer and on the bottom surface of the support layer.
Photomicrographs of cross-sections of the membrane indicated that all of
the membranes possessed the asymmetric structure characteristic of the
membranes of the invention.
Air flow measurements were also made on the membranes. Air flow rate
measurements were made using 25 mm disks cut from a pinhole-free area of
the membranes. Membrane samples were mounted in a 25 mm Swinnex filter
holder such that the skin layers faced upstream. Nitrogen gas was applied
to the upstream side of the membranes at a pressure of 69 kPa (10 psig)
and the flow of nitrogen through the membranes was measured using a
rotameter located downstream from the filter holder.
The air exposure times, pore diameters, and air flow rates are summarized
below in Table 3.
TABLE 3
______________________________________
Air flow
Exposure time Average pore dia. (.mu.m)
rates
Ex. (sec) Skin layer
Bot. surface
[L/(sec)m.sup.2 ]
______________________________________
24 0 0.5-1 50-100 307
25 5 0.5-1 50-100 310
26 15 0.5-1 50-100 307
27 30 <0.5 50-100 298
28 60 <0.5 50-100 297
29 120 <0.1 50 150
______________________________________
The data shows that the greater the air exposure time the lower the air
flow rate will be. This may be because the skin layer is less porous.
EXAMPLES 30-32
The procedure of Example 27 was repeated except that the membranes were
cast to various thicknesses. The resulting membranes were studied using
scanning electron microscopy. Photomicrographs of cross-sections of the
membranes indicated that all of the membranes possessed the characteristic
asymmetric structure of the membranes of this invention. Photomicrographs
were used to determine the average pore diameter in the skin layer and the
average pore diameter on the bottom surface of the membrane's support
layer. Air flow rates were measured for each membrane using the procedure
described in Examples 24-29.
The knife clearances, pore diameters, and air flow rates are summarized in
Table 4.
TABLE 4
______________________________________
Knife Average pore dia. (.mu.m)
Clearance Skin Bot. Air Flow
Ex. (mm) layer surface [L/(sec)m.sup.2 ]
______________________________________
30 0.13 <0.5 10-20 290
31 0.38 <0.5 100-300 272
32 0.51 <0.5 200-500 210
______________________________________
The data shows that the cast thickness affects the pore diameter on the
bottom surface of the support layer. The air flow rate may be decreasing
because the thickness of the skin layer increased as membrane thickness
increased.
EXAMPLES 33-37
The procedure of Example 18 was repeated except that in Examples 33 and 34
a 36 weight percent solution of calcium chloride dihydrate in methanol was
used to prepare a 3 weight percent casting solution of nylon 6,6 (Celanese
1200-1), and in Examples 35-37 the calcium chloride/methanol dihydrate
solution described above was used to prepare a 4 weight percent casting
solution of the nylon 6,6. In addition, various air exposure times were
used. The resulting membranes were studied using scanning electron
microscopy. The air exposure times, membrane structures, and average pore
diameters are summarized below in Table 5.
TABLE 5
______________________________________
Exposure time Average pore dia. (.mu.m)
Charact.
Ex. (sec) Skin layer
Bot. surface
structure
______________________________________
33 30 <0.1 100 yes
34 15 <0.5 50-100 no
35 0 1.0 100 yes
36 15 1.0 20-50 yes
37 30 1.0 100 yes
______________________________________
The data shows that at the 3 weight percent level of polyamide, and air
exposure time can affect whether the membrane will have the characteristic
asymmetric structure of the membranes of this invention. However, at the 4
weight percent level of polyamide, the exposure time did not affect
whether the membrane had the characteristic asymmetric structure.
EXAMPLES 38-41
The procedure of Example 18 was repeated except that various polyamides
were used instead of the NYCOA 589 CELANESE 1200-1 blend. The membranes
were studied using scanning electron microscopy. The polyamides used,
average pore diameters and membrane structures of the resulting membranes
are summarized in Table 6.
TABLE 6
______________________________________
Average pore dia.(.mu.m)
Charact.
Ex. Polyamide Skin layer
Bot. surface
structure
______________________________________
38 Nylon 6,6 0.2-3 20-50 yes
(ZYTEL 101)
39 Nylon 6 0.5-5 50-100 yes
(Nycoa 466)
40 Nylon 6 <0.1 50-100 yes
(Nycoa 589)
41 Nylon 6,9 <0.1 30-50 yes
(Aldrich
Chemical)
______________________________________
EXAMPLES 42-46
The procedure of Example 18 was repeated except that various salts and salt
concentrations were used instead of the 37 wt. % solution of calcium
chloride dihydrate and CELANESE 1200-1 alone instead of the nylon blend.
The resulting membranes were studied using scanning electron microscopy.
The salts used, percent concentration of the salt, average pore diameter
and membrane structures are summarized in Table 7.
TABLE 7
______________________________________
Average pore dia.
(.mu.m)
Salt Conc. Skin Bot.
Ex. structure Salt (%) layer surface
Charact.
______________________________________
42 CaCl.sub.2 29 <1.0 50 yes
43 Ca(NO.sub.3).sub.2.4H.sub. 2 O
46 <1.0 20-50 yes
44 Calcium 53 1.0 0.5-5 yes
Salicylate
45 CaCl.sub.2.4H.sub. 2 O
56 <0.5 10-20 yes
46 LiCl 24 1.0 100 yes
______________________________________
EXAMPLES 47-49
The procedure of Example 18 was repeated except that various blends of
isopropanol and methanol were used instead of methanol to prepare the
casting solution and an air exposure time of 5 seconds was used and
CELANESE 1200-1 alone was used instead of the nylon blend.
The resulting membranes were studied using scanning electron microscopy.
The average pore diameter, membrane structures, and percent volumetric
isopropanol/methanol blend used are summarized in Table 8.
TABLE 8
______________________________________
Average pore dia. (.mu.m)
Charact. isopropanol/
Ex. Skin layer
Bot. surface
structure
methanol
______________________________________
47 <0.1 30-50 yes 10/90
48 <0.1 50-100 yes 30/70
49 <1.0 50 yes 50/50
______________________________________
The data shows that blends of isopropanol and methanol can be used to
prepare the casting solution. However, blends containing more than 50
volume percent isopropanol would not dissolve the polyamide.
Various modifications and alterations of this invention will become
apparent to those skilled in the art without departing from the scope and
spirit of this invention.
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